Generally, the pointer instrument using the stepping motor is configured such that a rotation of the stepping motor is transmitted to a pointer using a speed-reduction mechanism including a plurality of gears, and accordingly the pointer indicates travel speed or engine speed.
In a case when a stepping motor is used as an instrument movement, unlike a vector follow-up method in which a magnet rotor follows in a synthesized-vector direction of a coil magnet field as in a cross coil instrument, variation of input signals are obtained in a certain period, and the motor is step-driven by an angle corresponding to the variation in many cases. A magnetic pole of a magnet rotor is subjected to micro-step processing performed in each step of a comb-like toothed yoke or in each interval between those steps so that the magnet rotor is driven. Therefore, in a typical configuration, a position of the magnet rotor in a situation when a power switch is turned on is assumed as an initial value (in a pointer instrument, zero initialization is performed, for example, by forcibly returning the rotor position to a zero position on a dial plate), and the motor is step-driven by an angle corresponding to a increase or decrease in input signal from the zero position.
Therefore, mechanical error may occur on the magnet rotor due to external vibration applied to a measuring instrument, which leads to a possibility that the magnet rotor is displaced from the precise angular position, which is established by drive control using step drive, that is, the magnet rotor strays from a precise position between the comb-like toothed yoke and the magnetic pole of the magnet rotor, resulting in an angular displacement by an angle corresponding to one step. Once such angular displacement so-called loss of synchronous occurs, a pointer cannot indicate the precise angular position, and the pointer is then always positioned at the angular position which includes the error angular due to the loss of synchronous unless the initialization is applied.
The phenomenon of loss of synchronous may not only occur due to the mechanical variation caused by external vibration, but also occur in the case that the magnet rotor cannot follow-up variation in input signal.
In such a stepping-motor instrument, a reset of loss of synchronous is performed to eliminate such pointing errors due to the loss of synchronous as many as possible. To perform the reset of the loss of synchronous, a stopper is mounted on a display board which is to be mounted with the stepping motor. In a measuring instrument that has been proposed, the stopper is protrusively provided at a position where a pointer starts rotation, and when the stepping motor falls into loss of synchronous, a reset-to-zero signal is outputted to the stepping motor so that the pointer is returned to a stopper side, and when the pointer comes into the stopper, reset of loss of synchronous is determined to be completed, and the reset-to-zero signal is stopped.
Patent document 1: JP-A-2001-314099